I am currently studying the textbook Light-Emitting Diodes (3rd Edition) by E. Fred Schubert. Chapter 2.1 Compound semiconductors says the following:
In 1874, Karl Ferdinand Braun (1874), a German scientist, reported on the electrical conduction in metal-semiconductor structures where the semiconductor was a metal sulfide such as ZnS, PbS, FeS$_2$, and CuFeS$_2$. He found that the current-voltage characteristic was asymmetric and non-ohmic and thus exhibited some degree of rectification. His work is the first demonstration of the rectifying properties of metal-semiconductor contacts and the first scientific research in compound semiconductors.
What is meant by "the current-voltage characteristic was asymmetric and non-ohmic", and why does this mean that the semiconductor exhibits some degree of rectification?